Three representative UK moorland soils show differences in decadal release of dissolved organic carbon in response to environmental change

M I Stutter,A P Rowland,D G Lumsdon

doi:10.5194/bg-8-3661-2011

Abstract

Abstract. Moorland carbon reserves in organo-mineral soils may be crucial to predicting landscape-scale variability in soil carbon losses, an important component of which is dissolved organic carbon (DOC). Surface water DOC trends are subject to a range of scaling, transport and biotic processes that disconnect them from signals in the catchment's soils. Long-term soil datasets are vital to identify changes in DOC release at source and soil C depletion. Here we show, that moorland soil solution DOC concentrations at three key UK Environmental Change Network sites increased between 1993–2007 in both surface- and sub- soil of a freely-draining Podzol (48 % and 215 % increases in O and Bs horizons, respectively), declined in a gleyed Podzol and showed no change in a Peat. Our principal findings were that: (1) considerable heterogeneity in DOC response appears to exist between different soils that is not apparent from the more consistent observed trends for streamwaters, and (2) freely-draining organo-mineral Podzol showed increasing DOC concentrations, countering the current scientific focus on soil C destabilization in peats. We discuss how the key solubility controls on DOC associated with coupled physico-chemical factors of ionic strength, acid deposition recovery, soil hydrology and temperature cannot readily be separated. Yet, despite evidence that all sites are recovering from acidification the soil-specific responses to environmental change have caused divergence in soil DOC concentration trends. The study shows that the properties of soils govern their specific response to an approximately common set of broad environmental drivers. Key soil properties are indicated to be drainage, sulphate and DOC sorption capacity. Soil properties need representation in process-models to understand and predict the role of soils in catchment to global C budgets. Catchment hydrological (i.e. transport) controls may, at present, be governing the more ubiquitous rises in river DOC concentration trends, but soil (i.e. source) controls provide the key to prediction of future C loss to waters and the atmosphere.

Highlights

Soils store large amounts of organic carbon (1500 PgC in the upper 1 m; Batjes, 1996), estimated as three-times and twice that in above ground biomass and atmospheric pools, respectively (Eswaran et al, 1993)
4.1 Soil Dissolved organic C (DOC) responses to environmental change factors
The divergent DOC concentration trends for these three soils at the Environmental Change Network (ECN) sites contrasts to a dominance of decreasing DOC concentration trends in the compiled soil solution literature (Table 1)

Summary

Introduction

Soils store large amounts of organic carbon (1500 PgC in the upper 1 m; Batjes, 1996), estimated as three-times and twice that in above ground biomass and atmospheric pools, respectively (Eswaran et al, 1993). Dissolved organic C (DOC) is a rapidly cycling component of the soil carbon (C) pool and an important indicator of change in soil C stability, and in the contributing biogeochemical processes. At catchment scales the magnitude of soil DOC release relates closely to the soil C pool size (Hope et al, 1997). Upland areas with C rich soils are deemed sensitive indicators of environmental change and DOC a key parameter to observe. The extent and dynamics of DOC release are governed by inter-related physico-chemical and biological soil conditions, including the nature of the soil organic matter itself (Kalbitz et al, 2000). DOC is a key component of C fluxes (Hope et al, 1997) and influences many ecosystem processes; mobilization, transport and bioavailability of nutrients, metals and organic contaminants (Lawlor and Tipping, 2003)

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Conclusion